Television transmitting system



All

R. D. KELL :inventor TELEVISION TRANSMITTING SYSTEM Filed Dec'. 24, 1940 Il"Ik Patented Feb. 29, 1944 TELEVISION TRANSMITTING SYSTEM Ray n. Keil, naadonnela, N. J., ssamm to nacio Corporation of America, a corporation o! Dela- Application December 24, 1940, Serial No. 371,514

Claims.

My invention relates to television transmitters vand particularly to transmitters in which the picture signals frequency modulate the carrier wave'.

It has been the practice to operate amplitudemodulated television transmitters in such a way that black in the picture signals will drive the power amplifier substantially to its continuous output rating while the synchronizing pulses drive it substantially to maximum power output, this being permissible because the synchronizing pulses occur for only a smallvpart of the total time and do not add greatly to the power that the anodes of the amplifier must dissipate. A transmitter which is operated in this way is described and claimed in my Patent 2,298,797, issued October 13, 1942, and entitled Television transmitters." While for a given tube complement, the use of frequency modulation gives a certain gain in the receiver output as compared with that obtained when using amplitude modulation, there is the disadvantage that, since the carrier amplltude is constant, the power amplifier output for synchronizing pulses is the continuous rating output rather than the maximumoutput. This fact offsets, to a certain extent, the advantages of frequency modulation. v

An object of the present vinvention is to provide an improved television4 or picture transmitter in which the advantages of frequency modulation are obtained without the disadvantage of not being able to drive the transmitter to maximum power output by the synchronizing signals.

A further object of the invention is to,'provide an improvedmethod of and lmeans for transmitvting television signals or the like.

In accordance with a preferred embodiment of the invention, the carrier wave is frequency-modulated by the picture signals only, the carrier wave amplitude having the maximum amplitude permitted by the continuous power rating of the power ampliiier tubes, and it is amplitude-modl the following description taken in connection withthe accompanying drawing in which Figure 1 is a circuit diagram of a television transmitter embodying my invention,

Figure 2 is a view illustrating the modulated carrier -wave which is radiated from the transmitter, and

- Figure 3 is a view showing the overall selectivity characteristic of a television receiver for receiving the signal transmitted from .the transmitter of Figure 1.

Referring to Figure 1, there is' shown a television transmitter comprising a carrier Wave oscillator I0 which may be of a well-known type having a tuned circuit Il, one end of which is coupled to the grid of the oscillator tube I3 by a condenser Il.

The output of oscillator I0 is frequency-modu- `1atea by the picture signin .indicated at s which The function of the reactance tube I9 is to vary the effective reactance across the'oscillator tuned circuit II in accordance lwith the voltage amplitude variations on the grid of the reactance tube. Thus a frequency-modulated carrier wave is obtained. In the particular example shown, the tube I9 is connected across the tuned circuit II by a connection from the plate of tube I9 to the plate of oscillator tube I3 and by a connection from the cathode of tube I9. to ground. A resistor 2| connected between the grid and plate of tube I9 through a blocking condenser 22 functions, together with the grid-cathode capacity indicated at 23, to provide the necessary out-ofphase component for the reactance tube operation.

A diode 26 is provided to reinsert the direct curlevel or blanking pulses are of positive polarity a-t the plate of diode 26 and will cause a ow oi.' diode current whenever they drive the diode plate positive with respect to its cathode. This puts a charge on coupling condenser I1. At the end of a pulse, the condenser I1 loses a` small part of its charge through aresistor 2Iwhereby the D.C. voltage across condenser Il may always have a value determined by the height of black level as measured from the A.C. axis of the signal. It is assumed, of course, that black in the picture has been made to go to a fixed level at a preceding point in the system, after which the D.C. component was lost.

The biasing battery 28 sets the voltage level at` which the diode 28 passes current. Battery 28 Aalso applies a negative biasto the control grid of 2 l asuma tube I9 so that the synchronizing pulses never drive the grid of tube I8 positive with respect to its cathode.

The inductance coil I8 is provided to separate the electrode capacities of tubes I9 and 2i in order to l'lold up the high video frequency response and, also. in order to keep the oscillator diated from an antenna 32.

The system described so far produces a carrier wave which is not modulated by synchronizing pulses. It is modulated only by picture signal and is of a constant amplitude which drives the power amplifier substantially to its maximum output as fixed by its power rating for continuous output.

In accordance with my invention, the synchronizing pulses, which are produced by a suitable synchronizing pulse generator 30, are put on the carrier wave by amplitude modulation. This may be accomplished by supplying the synchronizing pulses (for both line synchronizing and framing) to an amplifier or modulating tube 36 which is coupled to the grid circuit of the power amplifier 3| for grid modulation. This coupling, in the example illustrated, is through a coupling condenser 31 and a switch 38.

When grid modulation is employed, the amplifier 3l preferably is biased by means of a suitable voltage source 39 for class B operation. A coil 4I provides the necessary impedance between ground and the point at which the modulating voltage is applied.

As indicated at 42, the synchronizing pulses are applied to the modulator 3B with such polarity that they drive the control grids of the tubes in power amplier 3| more positive so as to momentarily increase their power output;

The modulated wave which is radiated from the transmitter isshown in Figure 2. It will be seen that up to a certain amplitude the carrier wave is frequency-modulated, this amplitude representing black in the picture infthe example shown, and that, beyond this amplitude, the carrier wave is amplitude-modulated. In Figure 2, the amplitude-modulated portionof the wave indicated at 42A has been produced by the horizontal or line synchronizing pulses.

It shouldA be understood that the amplitude modulation may be obtained by other methods than grid modulation. For example, by connecting the switch 38 to the conductor 48, the power amplifier 3| may be plate-modulated, as taught by Helsing. For plate modulation, the switch 41 is opened whereby the choke coil 48 is effectively connected into the amplifier plate circuit. When plate modulation is employed, the amplifier Il may be operated class C, to obtain maximum efficiency and maximum power output.

It will be understood that the synchronizing pulses and the picture signal occur in a fixed time relation with respect to each other in the usual manner, as' taught by the prior art, the line synchronizing pulses occurring at the end of each scanning line and the framing pulses occurring at the end of each neld or frame deflection.

Television receivers preferably are operated to give single side band reception as described in Grundmann Patent 2,261,803, issued November 4, 1941, and entitled Wide band amplifiers." Such receivers have the frequency response char..A

acteristlc shown in Figure 3. When receiving a carrier wave modulated according to the present invention, the receiver is tuned so that the carrier frequency fn is located half way down on the slopeof the low frequency side of the characteristic curve, as indicated.- The picture signals have been made to deviate or swing the carrier wave over the entire slope of the curve between the frequencies fr and fz. It may be noted that, when the receiver selectivity curve is drawn for the I. F. amplifier' with the response plotted against the I. F. signalitself, the curve is reversed with the carrier on the high side because the local oscillator frequency is so lo-- cated with respect to the incoming signal as to invert the frequency.

It is during the time the signal is deviated to the frequency fz representing black in the picture that the amplitude of the carrier wave is increased by a synchronizing pulse. Thus, there' frequency-modulated by the synchronizing pulses.

In the embodiment of the invention Just described, the oscillator output preferably is deviated between frequencies f1 and f2 (Fig. 3).. In this case, frequency fz represents a certain amplitude 0f synchronizing pulse. The desired additional amplitude of the synchronizing lpulse is obtained by the amplitude modulation.

I claim as my invention:

1. Thel method of operating a television transmitter havingv a power amplifier for amplifying a modulated carrier wave which comprises frequency-modulating said carrier wave with said wave at an amplitude which drives said amplifier substantially to its maximum continuous power output rating and amplitude-modulating said carrierA wave by periodically recurring synchronizing pulses.

2. The method of operating a television transmitter having an amplifier for amplifying a modulated carrier wave which comprises frequencymodulating said carrier wave with said wave at anamplitude which drives said amplier substantially to its maximum continuous power output rating and amplitude-modulating said can-ier Wave by periodically recurring synchronizing pulses which periodically drive said amplifier I substantially beyond said continuous power output rating.

3. The method of television transmission in a ysystem wherein at the receiver the frequency rei -f scams y v3 mitter having a power ampllner for amplifying a modulated carrier wave which comprises frequency modulating a carrier wave with picture" for producing a carrier wave which is to be modulated by picture signals and periodically recurring synchronizing pulses, means for frequency modulating said carrier wave by said picture signals and for producing a deviation to a frequency representing a predetermined'level such as black in the picture during the occurrence of said pulses', and means for amplitude modulating said carrier wave simultaneously by said synchronizi0 ing pulses.

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